Environmental Engineering Reference
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the concentration of gaseous atmospheric elemental mercury drops dramatically. Other po-
lar monitoring stations quickly reported the same observations (in the Arctic and Antarc-
tic). The greatest depletion is correlated with a corresponding reduction in surface air con-
centrations of ozone. What is going on? The chemical and physical processes involved re-
quire sunlight in order to perform their tricks. The gist of a rather technical explanation
is that over the winter, sea spray results in the injection of bromine into the atmosphere,
which, when the sun appears in spring, reacts with ozone (explaining its depletion at this
time). A string of short-lived intermediate reactive compounds are then formed involving
bromine, which lead to the conversion of elemental gaseous mercury into a reactive ion-
ic form that quickly deposits onto the snowpack. Mercury concentrations in the snowpack
can increase by 100 times during AMDEs, but it is thought that up to about 75% may be
reemitted back into the atmosphere as elemental mercury within a few days. One of the
unanswered questions is what happens to the remaining 25% because it is potentially avail-
able for conversion into biologically available methylmercury. Uncertainty on the signific-
ance of AMDEs to the overall budget of mercury gain and loss to the Arctic ecosystem is
at present a key research issue. For more information on AMDEs, take a look at the 2008
review by Alexandra Steffen and colleagues.
The AMAP 2011 assessment team looked at more than 80 data sets from localities at
higher latitudes than 60° north to examine trends of mercury levels in marine mammals,
marine fish, marine invertebrates, seabirds, freshwater fish and land mammals. They re-
ported “a recent increase in 16% of data sets, a recent decrease in 5% of data sets and
no change in or fluctuating trends in the remaining 79% of data sets”. As expected, bio-
magnification has led to a general linear trend of increasing concentrations with increasing
trophic level within any given food web. However, an interesting west to east trend was
seen with levels. Most of the species showing increasing levels from freshwater and mar-
ine environments were collected from Arctic Canada. Here, marine species were showing
levels increasing by 5% or more per year. In contrast, all species collected north of Europe
showedeithernotrendordecreasinglevels.TheFaroeIslands,IcelandandGreenlandwere
transitional between these two patterns. Even polar bears in Svalbard appeared to be carry-
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